KARKINOS (or Carcinus) was a giant crab which assisted the nine-headed serpent Hydra in its battle with Herakles at Lerna. The hero crushed it beneath his foot, but as a reward for its service, it was placed amongst the stars as the Constellation Cancer by Hera.

PARENTS

Nowhere stated

Pseudo-Apollodorus, Bibliotheca 2. 77 - 80 (trans. Aldrich) (Greek mythographer C2nd A.D.) :
"For his second labour Herakles was instructed to slay the Lernaian Hydra. The beast was nurtured in the marshes of Lerna . . . [Herakles attacked her and] she hung on to him by wrapping herself round one of his feet, and he was unable to help matters by striking her with his club, for as soon as one head was pounded off two others would grow in its place. Then a giant crab came along to help the Hydra, and bit Herakles on the foot. For this he killed the crab."

Plato, Euthydemus 297c (trans. Lamb) (Greek philosopher C4th B.C.) :
"[Plato uses the myth of the Hydra as a metaphor for urgument :] Herakles, who was no match for the Hydra . . . who was so clever that she sent forth many heads . . . in place of each one that was cut off; . . . [and a] crab . . . from the sea--freshly, I fancy, arrived on shore; and, when the hero was so bothered with its leftward barks and bites, he summoned his nephew Iolaus to the rescue, and he brought him effective relief."

Pseudo-Hyginus, Astronomica 2. 23 (trans. Grant) (Roman mythographer C2nd A.D.) :
"Cancer. The Crab is said to have been put among the stars by the favour of Juno [Hera], because, when Hercules had stood firm against the Lernaean Hydra, it had snapped at his foot from the swamp. Hercules, enraged at this, had killed it, and Juno [Hera] put it among the constellations."